PhD Oral Exam - Murilo Sergio Lamana, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The development of thermal barrier coating (TBC) systems is vital for enhancing gas turbine performance in aerospace applications, enabling higher operating temperatures and improved efficiency. The NiCoCrAlY bond coat, a critical component of TBC systems, ensures oxidation protection and robust adhesion between the ceramic topcoat and metallic substrate. The microstructure and oxidation resistance of the bond coat, governed by the deposition process, strongly influence TBC durability and functionality. This thesis examines thoroughly the Low-Temperature High-Velocity Air-Fuel (LT-HVAF) thermal spray process for depositing NiCoCrAlY bond coats, emphasizing its near-solid-state deposition characteristics, comparable to cold spray, and its potential for high-performance aerospace coatings.

The research investigates single-splat deposition and correlated in-flight oxidation behavior of LT-HVAF deposited bond coats, the effects of LT-HVAF torch geometry variation on the coating microstructure, and high-temperature isothermal oxidation performance. Single-splat studies were conducted to characterize the deposition dynamics of NiCoCrAlY particles, revealing splats with minimal in-flight oxidation due to the low thermal input and high particle velocities of LT-HVAF. This preserves the aluminum content within the coating matrix, essential for forming a continuous α-Al₂O₃ scale during high-temperature exposure. Advanced diagnostics, including real-time particle analysis using the DPV system and post-deposition characterization via scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and focused ion beam (FIB) imaging, were employed to establish correlations between process parameters, splat morphology, and coating microstructure.

The investigation into LT-HVAF torch design focused on varying nozzle lengths, with particle velocities measured using the DPV Evolution sensor to correlate with coating characteristics. Shorter nozzles resulted in reduced deposition efficiency due to decreased particle residence time in the flame, which exhibited lower heat transfer and rebounded without contributing to coating formation. Conversely, longer nozzles increased porosity due to limited plastic deformation of larger, non-molten or partially molten particles. However, shorter nozzles promoted denser coatings, likely due to the peening effect of larger particles. Compositional analysis revealed minimal oxygen content at lower magnifications using standard SEM and EDS, with nanoscale aluminum and yttrium oxide precipitates detected in FIB-produced cross-sections under ultra-high-resolution SEM and windowless EDS. The oxygen content and oxide layer thickness were similar for both nozzle configurations, suggesting limited heat exposure during deposition, characteristic of the i7A LT-HVAF system.

Isothermal oxidation tests at 1000°C compared LT-HVAF-deposited NiCoCrAlY coatings to cold-sprayed coatings, highlighting the advantages of LT-HVAF near-solid-state deposition. LT-HVAF coatings exhibited superior resistance to high-temperature degradation, forming stable internal oxide scales with minimal deterioration. Post-deposition short-term oxidation further enhanced coating homogeneity, reduced porosity, and stabilized β-phase grains, improving long-term oxidation performance.

This study elucidates critical relationships between LT-HVAF process parameters, torch configuration, feedstock characteristics, and coating microstructure, affirming the process’s potential for producing durable bond coats for aerospace turbines. The findings underscore the advantages of LT-HVAFs near-solid-state deposition, providing a foundation for future advancements in TBC systems and process optimization for next-generation gas turbine technologies.